The altitude of airplanes and other aircraft has traditionally been determined by means of an instrument based on a barometer. This is the so-called barometric altimeter. Although such altimeters have been widely used, they have their shortcomings. For example, they should be corrected for atmospheric pressure variations due to weather changes and measure height above mean sea level not height above the ground.
A radio altimeter is an altimeter which uses electromagnetic radiation to determine the altitude of the aircraft. Such an altimeter depends upon the time difference between the transmission of a radio signal and the reflected signal. U.S. Pat. No. 2,020,347 describes an early device. A radar altimeter is still another type of altimeter (see U.S. Pat. No. 3,543,270). Conventional radio altimeters generate signals at approximately 4.3 gigahertz (GHz). Although the radio altimeter or radar altimeter have advantages over the barometric altimeter, they cannot be used under all circumstances.
In particular, during aircraft missions over unfriendly terrain, it is often necessary to prevent detection of the aircraft by securing all sources of radio energy. Thus, radio transmitters, radars and radio altimeters are turned off to prevent the enemy from using their emissions for detection and weapons homing. Although barometric altimeters can be used under these circumstances, they do not have the desired degree of accuracy and responsiveness needed for such missions. They do not measure height above the ground.
Thus, it would be highly desirable for a means to be found to provide the advantages of a radio or radar altimeter, while not producing emissions which can result in the aircraft being detected. In effect, there is a need for a passive radio altimeter. Passive ranging devices are not unknown to the art. U.S. Pat. No. 4,130,360 teaches a laser altimeter. U.S. Pat. No. 4,558,323 describes a method of passive measurement of range, velocity and the course of a target relative to a test aircraft. However, the subject of passive altitude measurement has been neglected.
Clearly, a method and apparatus which would allow one to passively determine the altitude of one's own aircraft above the ground with the degree of accuracy expected from a radio altimeter would solve a difficult problem and satisfy a need long felt by military and commercial aviation.